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B.D. Moore (Rice Univ.), J.J. Hester (Arizona State Univ.), R.J. Dufour (Rice Univ.)
Analysis of the emission-line spectra of photoionized nebulae (e.g., H~{\sc ii} regions, PNe) is important to the study of the chemical evolution of galaxies. The emission lines observed in a given spectrum are translated into ionic abundances through a standard methodology based on ratios of diagnostic lines. It is inherently assumed that the ionization structure of the region is simple, consisting of two or three zones. These ionic abundances are then used to determine the elemental abundances in the region, usually through the use of ``ionization correction factors'' (ICFs). These ICFs, derived either empirically or from a grid of photoionization models, are intended to correct for the unobserved ionic stages of one element based on the observed ionic abundances of a different element or elements. The accuracy of the elemental abundances from spectra are often either assumed to be dominated by the observation error or ignored altogether. We examined the systematic errors of the methodology by analyzing the spectra of model nebulae. Tests of our analysis software on Cloudy models of isothermal slabs show an accuracy of ±0.025 dex for most ions of interest. Using a grid of constant-density H~{\sc ii} region models we construct the ICFs based on the abundances of O+/O++ and S+/S++. These ICFs are used to analyze the spectra of the constant-density models as well as models with exponentially-rising outer edges. For nitrogen and sulfur we find errors in elemental abundances that exceed by a factor of a few the accuracy of our analysis software. For secondary elements such as helium and argon the errors commonly exceed 0.1 dex, with the errors for neon as large as a factor of 3. The abundance errors in the exponential models were generally larger than those of the constant-density models. Models resembling bright knots show even larger discrepancies.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.